The present invention relates to a biaxially oriented laminated polyester film. More specifically, it relates to a biaxially oriented laminated polyester film which is excellent in film winding properties and handling properties and provides excellent electromagnetic conversion characteristics when it is used as a base film for high-density magnetic recording media, particularly digital magnetic recording media.
A biaxially oriented polyester film typified by a polyethylene terephthalate film is used for various purposes, particularly as a base film for magnetic recording media, thanks to its excellent physical and chemical properties.
In recent years, along with efforts made to increase the density and capacity of a magnetic recording medium, a base film having a more smooth surface and a smaller thickness has been desired. Particularly, a heavy-layer metal type magnetic tape having performance equivalent to a deposited magnetic tape has recently been developed and demand for a base film having a smooth surface has been growing more and more.
However, when the surface of a base film is smoothened to retain excellent electromagnetic conversion characteristics, its slipperiness or air squeezability deteriorates. As a result, when the film is rolled, it is apt to wrinkle or form debris, thereby making it extremely difficult to roll the film nicely. When the slipperiness of the base film is low in the film processing step, the friction of the base film with a metal roll in contact with the base film increases, whereby the film wrinkles, a magnetic layer cannot be applied well, or the film cannot be calendered well.
To improve the slipperiness of the polyester film, the following methods are employed: (i) one in which inert particles are deposited by the catalyst residue in a raw material polymer in the production process and (ii) one in which the surface of the film is finely uneven by adding inert particles or the like. Generally speaking, the greater the size and content of particles contained in the film, the more the slipperiness of the film improves.
As described above, the surface of the base film is desired to be as smooth as possible in order to improve electromagnetic conversion characteristics. When the surface of the base film is rough and a magnetic recording medium is formed using the base film, protrusions on the surface of the base film thrust up the surface of a magnetic layer after the application of the magnetic layer, thereby deteriorating electromagnetic conversion characteristics. In this case, the greater the size and content of particles contained in the base film, the more the surface of the film becomes rough, thereby deteriorating electromagnetic conversion characteristics.
To improve both slipperiness and electromagnetic conversion characteristics which are conflicting properties at the same time, there is widely known means of producing a laminated film consisting of two layers which differ from each other in surface roughness: the surface of one layer on which a magnetic layer is applied is made smooth to improve electromagnetic conversion characteristics and the surface of the other layer is made rough to improve slipperiness.
However, even when the above biaxially oriented laminated polyester film is used and the surface (to be referred to as xe2x80x9crough surfacexe2x80x9d hereinafter) opposite to the magnetic layer application surface is roughened, if the thickness of the base film is very small, a lubricant contained in the layer of the rough surface affects the layer of the magnetic layer application surface (smooth surface) according to its type, particle diameter and content and undulates the smooth surface, thereby worsening the smoothness of the smooth surface.
Particularly, in the latest high-density magnetic recording medium, a magnetic layer having a more smooth surface is desired, a metal calender having a high linear pressure is used, and a lubricant contained in the rough side layer has a greater bad influence upon the surface properties of the smooth surface by thrusting up the smooth surface.
To reduce the number of protrusions thrusting up the smooth side layer from the rough side layer, there have been proposed a method for reducing the particle diameter of a lubricant to be contained in the rough side layer and a method for containing a small amount of a lubricant having a large particle diameter. However, sufficient air squeezability cannot be obtained in the former case because the height of protrusions formed on the surface of the rough side layer is low, whereas sufficient film slipperiness cannot be obtained in the latter case because the density of protrusions formed on the surface is low. Further, when the film is rolled, it wrinkles longitudinally in the former case and debris is formed in the latter case, thereby making it impossible to obtain a sufficient product yield.
Meanwhile, to further smoothen the magnetic layer application surface of the film in order to improve electromagnetic conversion characteristics, there is proposed a smooth layer which contains substantially no lubricant. In this case, the running properties of the smooth surface are unsatisfactory in the tape processing step, the film wrinkles in that step, and the product yield greatly drops.
The present inventor has conducted intensive studies to develop a film which solves the above problems at the same time and has accomplished the present invention.
That is, it is an object of the present invention to provide a biaxially oriented laminated polyester film in which the particle diameters and amounts of lubricants to be contained in its rough side and smooth side are specified and the surface roughnesses of its rough side and smooth side and the friction coefficient of the film are specified.
It is another object of the present invention to provide a biaxially oriented laminated polyester film which is useful as a base film for high-density magnetic recording media and has excellent electromagnetic conversion characteristics and also excellent winding properties and running properties as a film.
It is still another object of the present invention to provide a magnetic recording medium comprising the biaxially oriented laminated polyester film of the present invention as a base film.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by a biaxially oriented laminated polyester film comprising:
(A) a polyester A layer (1) which contains at least the following three different types of inert fine particles in the following ratio:
(a) 0.001 to 0.1 wt % of first inert crosslinked polymer fine particles having an average particle diameter of 0.3 to 0.7 xcexcm and a relative standard deviation of particle size distribution of 0.5 or less,
(b) 0.05 to 0.4 wt % of second inert crosslinked polymer fine particles having an average particle diameter of 0.2 to 0.4 xcexcm and a relative standard deviation of particle size distribution of 0.5 or less; and
(c) 0.1 to 1.0 wt % of third inert fine particles having an average particle diameter of 0.01 to 0.3 xcexcm, the average particle diameter of the second inert crosslinked polymer fine particles being 0.1 to 0.4 xcexcm smaller than the average particle diameter of the first inert crosslinked polymer fine particles and 0.1 to 0.3 xcexcm larger than the average particle diameter of the third inert fine particles, the three different types of inert fine particles showing clearly distinguishable three particle size peaks existent within the above ranges of average particle diameter in a particle size distribution curve, and (2) which has a surface roughness WRa of 5 to 20 nm and a surface roughness WRz of 100 to 300 nm; and
(B) a polyester B layer formed on the polyester A layer and having a surface roughness WRa of 2 to 10 nm and a surface roughness WRz of 30 to 150 nm, wherein
(C) the static friction coefficient between the exposed surface of the polyester A layer and the exposed surface of the polyester B layer is 0.5 or less.
Secondly, the above objects and advantages of the present invention are attained by a magnetic recording medium comprising the biaxially oriented laminated polyester film of the present invention and a magnetic layer formed on the polyester B layer of the biaxially oriented laminated polyester film.